Active Pt-Nanocoated Layer with Pt-O-Ce Bonds on a CeO x Nanowire Cathode Formed by Electron Beam Irradiation.

Abstract

A Pt-nanocoated layer (thickness of approx. 10–20 nm) with Pt–O–Ce bonds was created through the water radiolysis reaction on a CeOx nanowire (NW), which was induced by electron beam irradiation to the mixed suspension of K2PtCl4 aqueous solution and the CeOx NW. In turn, when Pt-nanocoated CeOx NW/C (Pt/C ratio = 0.2) was used in the cathode layer of a membrane electrode assembly (MEA), both an improved fuel cell performance and stability were achieved. The fuel cell performance observed for the MEA using Pt-nanocoated CeOx NW/C with Pt–O–Ce bonds, which was prepared using the electron beam irradiation method, improved and maintained its performance (observed cell potential of approximately 0.8 V at 100 mW cm–2) from 30 to 140 h after the start of operation. In addition, the activation overpotential at 100 mA cm–2 (0.17 V) obtained for MEA using Pt-nanocoated CeOx NW/C was approximately half of the value at 100 mA cm–2 (0.35 V) of MEA using a standard Pt/C cathode. In contrast, the fuel cell performance (0.775 V at 100 mW cm–2 after 80 h of operation) of MEA using a nanosized Pt-loaded CeOx NW (Pt/C = 0.2), which was prepared using the conventional chemical reduction method, was lower than that of MEA using a Pt-nanocoated CeOx/C cathode and showed reduction after 80 h of operation. It is considered why the nanocoated layer having Pt–O–Ce bonds heterogeneously formed on the surface of the CeOx NW and the bare CeO2 surface consisting of Ce4+ cations would become unstable in an acidic atmosphere. Furthermore, when a conventional low-amount Pt/C cathode (Pt/C = 0.04) was used as the cathode layer of the MEA, its stable performance could not be measured after 80 h of operation as a result of flooding caused by a lowering of electrocatalytic activity on the Pt/C cathode in the MEA. In contrast, a low-amount Pt-nanocoated CeOx NW (Pt/C = 0.04) could maintain a low activation overpotential (0.22 V at 100 mA cm–2) of MEA at the same operation time. Our surface first-principles modeling indicates that the high quality and stable performance observed for the Pt-nanocoated CeOx NW cathode of MEA can be attributed to the formation of a homogeneous electric double layer on the sample. Since the MEA performance can be improved by examining a more effective method of electron beam irradiation to all surfaces of the sample, the present work result shows the usefulness of the electron beam irradiation method in preparing active surfaces. In addition, the quantum beam technology such as the electron beam irradiation method was shown to be useful for increasing both performance and stability of fuel cells.